This application relates generally to disc drive data storage devices and more particularly to an apparatus and method of writing servo track information thereon.
Disc drives are the most common means of storing electronic information used today. Typical disc drives have one or more magnetic media discs attached to a spindle; the spindle and discs are rotated at a constant velocity by a spindle motor. An actuator assembly, attached to a bearing shaft assembly next to the discs, radially traverses over the surface of the discs. The actuator assembly has a plurality of actuator arms, each with one or more flexures extending from the end of each actuator arm. A read/write head is attached to the distal end of each flexure. The actuator assembly is rotated about the bearing shaft assembly by a servo positioner. The servo positioner receives signals from a controller, rotates the actuator assembly, and positions the read/write head relative to the disc surface.
Information is transferred to and from the discs by the read/write heads attached to the flexures at the end of the actuator arms. Each head includes an air bearing slider that enables the head to fly on a cushion of air in close proximity to the corresponding surface of the associated disc. Most heads have a write element and a read element. The write element is used to store information to the disc, whereas the read element is used to retrieve information from the disc.
Discs, to facilitate information storage and retrieval, are radially divided into concentric circles known as xe2x80x9cservo tracksxe2x80x9d or xe2x80x9ctracksxe2x80x9d. Tracks are given a track number, among other identifying information, so that the servo positioner can align the read/write head over desired track. Information is stored or retrieved from the disc after the read/write head is in the correct position. The process of switching between different tracks is called xe2x80x9cseekingxe2x80x9d, whereas remaining over a single track while information is stored or retrieved is called xe2x80x9cfollowingxe2x80x9d.
Each track is linearly subdivided into pie-shaped sections, called xe2x80x9csegmentsxe2x80x9d or xe2x80x9csectorsxe2x80x9d. The two most common types of sectors are informational data sectors and servo data sectors. In a typical disc drive, the informational data sectors usually contain information generated or stored by the user such as programs files, application files, or database files. There may be ten to a hundred, or even more, informational data sectors dispersed around a single track.
The servo sectors, on the other hand, contain information that is used by the servo positioner to determine the radial, and linear, position of the head relative to the disc surface and relative to the track center. Servo sectors typically consist of a Grey code field, which provides coarse position information to the servo positioner such as the track and cylinder number, and a servo burst field, which provides fine position information to the servo positioner such as the relative position of the head to the track center. Generally speaking, the burst field creates a positive voltage on one side of the track centerline and a negative voltage on the other side of the track centerline. The read head can be aligned directly over a track centerline by positioning the read head such that the sum of the burst field voltages equal zero.
Servo sectors are usually placed between adjacent informational data sectors on the same track. A clock signal mechanism is used to insure that data intended to be stored in a servo sector does not overwrite data in an information sector (and vice versa).
During the servo writing process, a timing pulse from the clock signal mechanism notifies the servo positioner when the head is over a servo sector (as opposed to over an information sector). The write enable;signal is turned on and information is written to the servo sector. The timing pulse also notifies the servo positioner when the head is over an information sector. The write enable signal is turned off and servo information is not stored in the informational data sector during the servo writing process.
In contrast during, normal disc drive operation, the timing pulse notifies the servo positioner when the head is over an information sector (as opposed to a servo sector). The write enable signal is turned on, and data is written to the information sector. The timing pulse also notifies the servo positioner when the head is over a servo sector. The write enable signal is turned off and user data is not stored in the servo sector during normal disc drive operation.
The number of tracks located within a specific area of the disc is called the xe2x80x9ctrack densityxe2x80x9d. The greater the number of tracks per area, the greater the track density. The track density may vary as the disc is radially traversed. Disc manufacturers attempt to increase track density in order to place more information on a constant size disc. Track density may be increased by either decreasing the track width or by decreasing the spacing between adjacent tracks.
An increase in track density necessitates increased positioning accuracy of the read/write elements in order to prevent data from being read from or written to the wrong track. Manufacturers attempt to fly the read/write head elements directly over the center of the desired track when the read/write operation occurs to insure that the information is being read from and written to the correct track. Hitting the track center target at high track densities requires that the tracks be as close to perfectly circular as possible when written to the disc surface.
Tracks are usually written on the disc during disc drive manufacturing using one of two means: 1) a servowriting machine, or 2) self-propagated servo writing. In both methods, a timing clock is used to determine when the head is over an area where a servo sector is to be written. A write enable signal is activated and servo information is written when the timing pulse indicates that the head is located over a servo sector. The write enable signal is de-activated and information is not written once the head exits the area where a servo sector is to be written.
A servowriting machine is a large piece of external equipment that writes servo tracks on a disc drive. The servowriting machine uses a very accurate lead screw and laser displacement measurement feedback device to precisely align a write element. The write element, which is attached to an external head/arm positioner, is aligned relative to where the desired track is to be written on the disc surface. A track is written on the disc once the write element is correctly aligned. The head/arm positioner then moves the write element a predetermined distance to the next desired track location. The head/arm positioner, therefore, controls both the track placement and track-to-track spacing.
A servowriter has several drawbacks. First, a typical disc may contain more than 60,000 servo tracks. The process of aligning and writing each track on the disc is very time consuming and expensive. Second, although very accurate at lower track densities, the servowriter cannot meet the accuracy requirements dictated by higher track densities. Finally, the servo writer must be used in a clean room because the disc components are exposed during servo writing; again adding to the expense of the servo writing process.
The second means of writing tracks on a disc is called self-propagating servo writing. Oliver et al first described this method of servo track writing in U.S. Pat. No. 4,414,589. Several other patents have disclosed slight variations in the Oliver patent, but the same basic approach is used. Under the basic method, the drive""s actuator assembly is positioned at one of its travel-range-limit stops. A first reference track is written with the write head element. The first reference track is then read with the read element as the head is radially displaced from the first reference track. When a distance is reached such that the read element senses a predetermined percentage of the first reference track""s amplitude, a second reference track is written. The predetermined percentage is called the xe2x80x9creduction numberxe2x80x9d. For example, the read element senses 100% of the first reference track""s amplitude when the read element is directly over the first reference track. If the reduction number is 40%, the head is radially displaced from the first reference track until the read element senses only 40% of the first reference track""s amplitude. A second reference pattern is written to the disc once the 40% is sensed by the read element. The head is then displaced in the same direction until the read head senses 40% of the second reference track""s amplitude. A third reference track is then written and the process continues. The process ends when the actuator arm""s second travel-range-limit stop is reached and the entire disc surface is filled with reference tracks. The average track density is then calculated using the number of tracks written and the length of travel of the head.
If the average track density is too high, the disc is erased, the reduction number is lowered so that a larger displacement occurs between tracks, and the process is repeated. If the track density is too low, the disc is erased, the reduction number is increased so that a smaller displacement occurs between tracks, and the process is repeated. If the track density is within the desired range, the reduction number for the desired average track density has been determined, the disc is erased, and servo tracks are written to the disc by alternatively writing servo and reference tracks. The servo tracks are further divided by alternatively writing servo and informational sectors.
A well-known problem with self-propagating servo writing is called xe2x80x9cradial error propagationxe2x80x9d. The servo system, when writing a new track during self-propagating servo writing, obtains position information by monitoring the signal generated in the read head by the previous track""s servo information. The servo system xe2x80x9cfollowsxe2x80x9d the path of the previous track, and therefore, the track being written inherits any imperfections (caused by spindle wobble, disc slip, changing head fly height, and thermal expansion among others) in the track being followed. The imperfections of the followed track may even be amplified within the written track if the closed loop gain of the servo positioner is larger than unity at certain frequencies.
Ideally, tracks are perfectly circular and spaced at a specific distance from each other. The imperfections in track shape and track spacing are referred to as xe2x80x9ctrack squeezexe2x80x9d. Track shape imperfections are referred to as dynamic or AC track squeeze, whereas track spacing imperfections are referred to as static or DC track squeeze. AC track squeeze refers to the situation in which two adjacent tracks have shape imperfections at different locations around their individual circumferences. The two tracks may be too close together at some points and too far apart at other points. DC track squeeze, on the other hand, refers to the situation in which two adjacent tracks are either closer or farther apart than a nominal distance. In other words, the spacing between the two tracks is incorrect even though the two tracks may be perfectly circular. The term xe2x80x9ctrack squeezexe2x80x9d is often used to generally refer to the combination of AC and DC track squeeze. Furthermore, the track-to-track variation of track shape is called the xe2x80x9crelative track shape errorxe2x80x9d, whereas the deviation of the track shape from a perfect circle is called xe2x80x9cabsolute track shape errorxe2x80x9d. The prior art methods of machine servo writing and self-propagated servo writing cannot achieve the accuracy needed for higher track densities because of inherent limitations in controlling track squeeze, relative track shape error, and absolute track shape error.
Yarmchuk et al in U.S. Pat. No. 5,659,436 extensively studied radial error propagation. Yarmchuk proposed that indefinite growth of written in errors are avoided by insuring that the propagation gain is less than unity at all frequencies. Yarmchuk proposed that the gain could be maintained at a value less than unity at all frequencies by carefully choosing the open loop transfer function and/or providing an appropriate reference correction table derived from the position error signal during the write revolution of the previous track. However, the method proposed by Yarmchuk fails to discuss the influence of measurement noise and requires complicated calculations for implementation.
The Yarmchuk method contains an additional drawback. Yarrnchuk allows relatively large absolute track shape inaccuracy (i.e., the deviation of the track shape from a perfect circle). In effect, the accuracy obtained by the Yamichuk method is the track following accuracy of the disc drive, or 10%.
Zero Acceleration Path (xe2x80x9cZAPxe2x80x9d) correction is another approach created to eliminate radial error propagation. The basic idea of ZAP correction is to add appropriate correction factors to the measured head position at each servo sector. The correction factors cancel all written in errors, thereby improving the shape of the modified track. The correction factors are typically determined during or after the servo track writing process. The correction factors are then written back on the discs; usually each servo sector has a dedicated field for storing the correction factors.
The prior art method of determining ZAP correction factors is called xe2x80x9cinverse transformationxe2x80x9d. Inverse transformation guarantees that track squeeze is minimized and that the tracks are circular. In other words, the inverse transformation method guarantees that the relative track shape error (the track-to track variation) remains small, and that the absolute track shape (the deviation of the tracks from a perfect circle) also remains small as the self- servo track writing propagates. The major disadvantage of using inverse transformation is that several disc revolutions are required to accurately determine the correction factors. Typically more than eight revolutions are necessary to achieve acceptable accuracy in today""s disc drives. An increase in track density requires an increase in the accuracy of the ZAP correction factors. Doubling the track density, for example, requires doubling the accuracy of the ZAP correction factors. The number of averaging revolutions must be four times higher to double the accuracy of the ZAP correction factors. Therefore, the total servo writing time will be eight times higher if track density is doubled because twice as many tracks (requiring four times the revolutions for accurate ZAP correction factors) are present on the disc. Each revolution increases the time and cost of servowriting.
Accordingly there is a need for a method of eliminating propagation of radial errors during self servo track writing that overcomes the limitation of prior art approaches.
Against this backdrop the present invention has been developed. This invention proposes a method that eliminates the growth of track shape errors during self-propagating servo track writing. The method insures that both the relative track shape error and that the absolute track shape error remains small. In other words, the track-to-track variation and the track variation from a perfect circle remain small. The present invention eliminates relative and absolute track shape errors by writing Zero Acceleration Path (xe2x80x9cZAPxe2x80x9d) correction factors into the servo sectors during track propagation. The present invention only requires two disc revolutions for each propagation step. Although described herein with reference to a magnetic media data storage device, it is to be understood that the invention may also be applied to other types of data storage devices.
A new ZAP correction method is presented, which is called xe2x80x9crecursive estimationxe2x80x9d. Recursive estimation guarantees that track squeeze is minimized, but it does not guarantee that the tracks are circular. In other words, the recursive estimation method guarantees that the relative track shape error (the track-to track variation) remains small, but recursive estimation does not guarantee that the absolute track shape (the deviation of the tracks from a perfect circle) remains small as self-servo track writing propagates. Therefore, the xe2x80x9crecursive estimationxe2x80x9d ZAP method is combined with the xe2x80x9cinverse transformationxe2x80x9d ZAP method in the present invention. A combination of recursive estimation and inverse transformation provides small absolute track shape error and small relative track shape error. Furthermore, a combination of recursive estimation and inverse transformation reduces the number of disc revolutions necessary to determine the ZAP correction factors for each track.
In a preferred embodiment of the present invention, the actuator is placed against one of its travel-limit-stops and a first servo track is written to the disc. Alternatively, the first track (or first several tracks in a servo track guide pattern) can be written on a conventional servowriting machine. ZAP correction factors are determined using inverse transformation and written into the first track, before the self-propagating process starts. Alternatively, if a servowriter is used, ZAP correction factors are determined by inverse transformation and written to the first track or the first several tracks of the servo track guide pattern. Inverse transformation requires several revolutions per track to determine the ZAP correction factors. The amount of time spent determining the ZAP correction factors for the initial tracks, however, is tolerable because inverse transformation is applied only to a small number of tracks. Self-propagating servo writing commences after the ZAP correction factors are determined and written to the initial track or tracks.
The head disc assembly (xe2x80x9cHDAxe2x80x9d) is connected to an electrical control system for self-propagating servo writing. The HDA is the combination of a magnetic media disc (or discs) and an actuator assembly. The control system activates the read element of the actuator assembly and displaces the actuator arm until the read head is aligned over the initial servo track. The control system reads and xe2x80x9cfollowsxe2x80x9d the ZAP corrected path of the initial servo track. The control system then activates the write element of the actuator assembly and a new servo track is written onto the disc. ZAP correction factors are determined and written to the servo sectors of the newly written servo track. Next, the control system displaces the actuator arm until the read element is aligned over the new servo track. The control system reads and xe2x80x9cfollowsxe2x80x9d the ZAP corrected path of the new servo track. The control system activates the write element and a new servo track is written. Again, ZAP correction factors are determined and written to the servo sectors of the newly written servo track. Again, the control system displaces the actuator arm until the read element is aligned over the new servo track. The process continues until the disc is filled with servo tracks; the write element writing a new servo track as the read element follows the previously written servo tracks.
In accordance with the preferred embodiment of the present invention, the ZAP correction factors for the self-propagated servo tracks are determined by using two disc revolutions per track; a xe2x80x9cwrite revolutionxe2x80x9d and a xe2x80x9ccorrection revolutionxe2x80x9d. The read element follows track k, the previously written track, and the write element writes servo marks for track k+1 during the write revolution. The position signal is monitored and recorded in order to estimate the shape of track k+1 according to the inverse transformation method during the correction revolution. The preferred embodiment of the present invention does not require extra disc revolutions, and therefore, it does not increase the time required for the self-propagating servo writing process. The present invention, in addition to other benefits, eliminates radial error propagation, takes into account the influence of measurement noise, does not require complicated calculations for implementation, and provides greater absolute track shape accuracy.